GSM A5/1: (Sub)Standard Security pt.2

GSM is a widely deployed standard for cellular communications, including security aspects. This post will describe one aspect of the GSM security architecture, how GSM security has been hacked and why.

The GSM standard deals with two main security concerns - payment and privacy. The first goal is to ensure that the person making a call pays for it. The second goal is to prevent unauthorized parties from accessing communications over the GSM network. This post will concentrate on the second area - privacy.

Cell phone bug?

The initial GSM standard, published in 1990, stipulates the usage of an algorithm called A5/1 for scrambling GSM voice communications. A5/1 has two important characteristics: it uses a 64-bit key and was intended to be kept secret.

Keeping an algorithm implemented by dozens of device manufacturers secret is good for as long as it lasts - which isn't very long. A5/1 remained secret for a few years, but was fairly quickly reverse engineered and was published on the Internet in 1999.

Cryptanalysts found several weaknesses in the A5/1 algorithm - but none as significant as the fact that the algorithm uses a 64-bit key.

Using a 64-bit key to encrypt data is fine as long as one of the following conditions is true:

1. You're living in the 20th century.
2. You're living in the early 21st century and the data secured by any specific key is not very valuable and there is no single known plain-text encrypted with each key